Various tests may be performed on a sample for diagnostic purposes, routine care, research, etc. In the medical field, the sample may include, for example, blood, urine, DNA, feces, tissue, etc. In other fields, such as bio-terrorism, samples may include, for example, air, water, soil, etc. Frequently, as part of the test, the sample may be mixed with a reagent to produce an outcome. In an in vitro setting, the mixing may take place outside of a patient's body, for example. Additionally, the different tests may require different quantities of a sample, which may be collected in various containers. In some instances, the container may have a sample capacity greater than is necessary for a particular test. It may be desirable to know the quantity of sample in a container, for example, to determine an appropriate amount of reagent to add, to make appropriate calculations, to act as an automated confirmation of the liquid levels, or to meet regulatory requirements.
The inventors of the present invention have determined that an existing problem with conventional automated and non-automated liquid level sensing for in vitro diagnostics is that the conventional methods may be complex, and therefore expensive. Additionally, conventional liquid level sensing methods may be unreliable in detecting increasingly smaller volumes, particularly without destruction of, or physical contact with, the samples. For example, the conventional visual method whereby an observer views a sample and records the liquid level of the sample may be inaccurate and time consuming.
The electro-capacitive method may be another conventional method for sensing a liquid level of a sample. With the electro-capacitive method a probe may be inserted into the sample, thereby contacting the sample, which may be destructive to the sample to some degree. Additionally, contact with the sample may increase the possibility of carryover between the different samples being tested. In other words, some of a first sample tested may remain on the probes used with the capacitive method, for example, and be carried-over to the second sample being tested by the same probes. The electro-capacitive method detects the level of the sample by measuring the position of the probe at the moment of contact with the sample that in turn produces a change in capacitance on the probe. However, the electro-capacitive method may be limited by the difficulty in detecting very small changes in capacitance, as well as unavoidable bulk parasitic changes and interferences in the environment when detecting the liquid level of the sample (ie. foam or dust on the surface of the sample; or turbulence in the sample).
Pressure sensors may be another conventional way to sense the liquid level of a sample. Pressure sensors measure the pressure of the sample wherever the pressure gauge is located. Similar to the probes used in the electro-capacitive method, the components of the pressure sensors may contact the sample and thereby destroy the sample or result in cross-contamination. Additionally, pressure sensors may have difficulty in detecting the liquid level due to interference, such as foreign objects or solids, in the sample, which may interfere with the sensors.
Another conventional way to sense the liquid level of a sample may be to use ultrasound, whereby a sound pulse is sent into the sample and a sensor examines time for an echo to return. Unlike electro-capacitive methods and pressure sensors, with ultrasound, no contact with the sample is necessary. However, ultrasonic level sensing may be difficult to implement, as it depends on air properties that may change with temperature and humidity. Additionally, the resolution associated with ultrasonic level sensing may be fairly low. Accordingly, a need exists for an improved method and apparatus for determining the amount of sample in a container.